One of the steps in making glass optical fiber by the vapor axial deposition (VAD) method is the step of depositing glass particles or "jacket soot" on a glass core rod. This is done by supplying to a torch glass raw material and fuel such that the torch projects the vaporized glass raw material within a flame toward the glass rod. The flame causes the raw material to react such as to deposit glass particles or glass soot on the core rod. The core rod typically extends vertically with the initial deposition at its upper end. It is then moved vertically and rotated so that glass soot is deposited along its entire length and circumference. After deposition, the soot surrounding the core rod or the "soot boule" is consolidated or sintered in a furnace to form a glass preform. The preform is typically elongated and thereafter is used as a source from which optical fiber is drawn.
Optical fiber is very widely used and, as a consequence, significant efforts have been made to reduce production costs. The step of making the jacket layer as described above can result in cracking of the soot boule, which may render the entire structure useless. There is therefore a continuing need for increasing the yield of methods for making optical fiber preforms without significantly increasing the cost or the time required for making such preforms.